1. Field of the Invention
This invention relates to a semiconductor device and particularly relates to a semiconductor device which having an improved electrostatic breakdown voltage.
2. Description of the Related Art
FIG. 23 is a circuit diagram of a compound semiconductor switching circuit device. The source electrodes (or drain electrodes) of first and second FET's, FET1 and FET2, are connected to a common input terminal IN, the gate electrodes of FET1 and FET2 are respectively connected to first and second control terminals Ctl-1 and Ctl-2 via resistors R1 and R2, and the drain electrodes (or source electrodes) of FET1 and FET2 are connected respectively to first and second output terminals OUT1 and OUT2. The control signals applied to first and second control terminals Ctl-1 and Ctl-2 are complementary signals and the FET to which the H level signal is applied turns ON and transmit the input signal applied to common input terminal IN to one of the output terminals. Resistors R1 and R2 have the resistance of 10 KΩ respectively and are provided for preventing high-frequency signals from leaking via the gate electrodes to the DC potential of control terminals Ctl-1 and Ctl-2 which are AC grounded.
FIG. 24 shows an example of a compound semiconductor chip having this compound semiconductor switching circuit integrated.
FET1 and FET2, which perform switching, are disposed at central parts of a GaAs substrate and the resistors R1 and R2 are connected to the respective the gate electrodes of the FET's. Also, pads, respectively corresponding to the common input terminal IN, the output terminals OUT1 and OUT2, and the control terminals Ctl-1 and Ctl-2, are disposed at peripheral parts of the substrate. A gate metal layer (Ti/Pt/Au) 20, which is formed at the same time as the forming of the gate electrodes of the respective FET's, includes a second-layer wiring, indicated by dotted lines, and a pad metal layer (Ti/Pt/Au) 30, which connects the respective elements and form the pads, includes a third-layer wiring, indicated by solid lines. An ohmic metal layer (AuGe/Ni/Au), which is in ohmic contact with the first-layer substrate, forms the source electrodes and drain electrodes of the respective FET's and forms electrodes at the ends of the respective resistors, and this layer is not illustrated in FIG. 11 as it overlaps with the pad metal layer FET1, shown in FIG. 24, are formed on an operating region 12, which is surrounded by alternate long and short dash lines. The three-teeth-comb-shaped third-layer pad metal layer 30 parts, which extend from the lower side, form a source electrode 13 (or drain electrode) connected to the output terminal OUT1, and below this is disposed a source electrode 14 (or drain electrode) formed by the first-layer ohmic metal layer. Also, the three-teeth-comb-shaped third-layer pad metal layer 30 parts, which extend from the upper side, form a drain electrode 15 (or source electrode) connected to the common input terminal IN, and below this is disposed a drain electrode 14 (or source electrode), formed by the first-layer ohmic metal layer. These electrodes are disposed in the form of engaged comb teeth and in between these, a gate electrode 17, formed from second-layer the gate metal layer 20, is disposed on the operating region 12 in the form of five comb teeth. The drain electrode 15 (or source electrode) that has the central comb tooth extending from the upper side is used in common by FET1 and FET2 to contribute to size reduction. Here, the gate width, which is the total gate width of the comb-shaped gate electrode 17 of each FET, is 600 μm.
FIG. 25 shows the results of measuring the electrostatic breakdown voltage of the switching circuit device shown in FIG. 24. Here, the electrostatic breakdown voltage is measured under the following conditions. That is, after applying a test voltage to the terminals of a test capacitor of 220 pF and thereby charging electricity in the test capacitor, the wiring for voltage application is cut off. Thereafter, the charged electricity in the test capacitor is discharged between the terminals of a tested element (FET) without resistance element or inductance element inserted. Thereafter it is determined whether the FET has broken down. If the FET does not break down, the test is repeatedly carried out with the application voltage being increased by 10V at a time, and the first application voltage at which the FET breaks down is measured as the electrostatic breakdown voltage.
As shown in FIG. 25, since there has been no attempt in the art to improve the electrostatic breakdown voltage, the electrostatic breakdown voltage is poor and only 140V between the common input terminal IN and the control terminal Ctl-1 and between the common input terminal IN and the control terminal Ctl-2.
Also, there is a variation of the electrostatic breakdown voltage depending on the terminals which the electrostatic breakdown voltage is measured between. Though the specific mechanism that determines this electrostatic breakdown voltage is unclear, with a switching circuit device, the minimum electrostatic breakdown voltage between two terminals is generally of the level of 100V or less as mentioned above, and the finest care is required for handling. That is, the minimum electrostatic breakdown voltage among the terminals of a device determines the electrostatic breakdown voltage of the device as a whole, and thus improvement of the minimum electrostatic breakdown voltage among these terminals is required.
Besides, unlike other devices for audio, video, and power supply applications, microwave communication devices are low in the internal Schottky junction or pn junction capacitance, and these junctions are weak against static electricity.
Generally in order to protect a device against static electricity, an electrostatic breakdown protecting diode may be connected in parallel between the terminals of a pn junction or Schottky junction, which is damaged readily by electrostatic discharge. However, this approach could not be applied to a microwave device since increased parasitic capacitance due to connection of a protecting diode causes degradation of the high-frequency characteristics.